Field of the Invention
This invention relates to light sources, and in particular, it relates to light sources based on wavelength conversion.
Description of the Related Art
Traditional light sources, such as fluorescent light, incandescent light, ultra-high performance lamps and xenon lights, cannot achieve high efficiency and long life. With the development of solid state light sources, light emitting diodes (LED) and semiconductor laser are gaining wide use in illumination device and display device markets.
There are typically two ways of generating a white light or a color light: one method directly uses color light sources such as red, green and blue LEDs to provide color lights, or mixes these color lights to generate a white light; the other method is based on wavelength conversion, which uses an excitation light to excite wavelength conversion materials to generate color lights, and further generates a white light by mixing the excitation light and/or the converted color light. For example, a short wavelength light such as blue or UV light may be used as excitation light to excite wavelength conversion materials, which may be but are not limited to phosphors. Using green light as an example, current green LED or green laser cannot achieve high efficiency and are expensive. On the other hand, solid state light sources for blue and UV light have high efficiency and low cost; therefore, using the second method described above to generate green light has a large potential.
Using phosphor as an example, conventional light sources that use LED chip and wavelength conversion method typically coat the phosphor material on the surface of the LED chip. A problem with this is that when the converted light passes through the phosphor layer, a part of it is scattered back to the LED chip; because the LED chip absorbs a part of this light, the absorbed part of the converted light will not be output by the light source, resulting in a loss. Further, the heat generated by the LED chip and the heat generated by the wavelength conversion material interfere with each other, resulting in a reduction in the light emitting efficiency of the LED chip and the conversion efficiency of the wavelength conversion material. This can shorten the life of the LED device. For these reasons, this type of light source cannot achieve high brightness.
U.S. Pat. No. 7,070,300 discloses a method that separates the LED chip and the phosphor material to solve the above problem. As shown in FIG. 1, the excitation light from one or more LEDs 102 is collimated by a collimating device 108. A dichroic filter 110 reflects the excitation light to another collimating device 114, which focuses the excitation light onto the phosphor plate 112. The converted light from the phosphor plate 112 is transmitted through the dichroic filter 110 to become the output light. In this configuration, the converted light will not be scattered back to the LED chip, and the heat generated by the LED chip and the wavelength conversion material will not interfere with each other. The dichroic filter plate 110 separates the optical paths of the excitation light and the converted light, which facilitates the extraction of the converted light. As a result, a substantial majority of the back-scattered excitation light or converted light are recycled, thereby solving the above-described problems of the conventional technology. However, this design has its own problems: because current LED chips typically have a square or rectangular shape, its output light has a Lambertian distribution; it is difficult to design and produce highly efficient optical modules to collect and collimate the light from the LED chip. This difficulty partly prevents the commercialization of this technology. Further, because of the light separation by the dichroic filter 110, the excitation light cannot be output as a part of the output light of the light source. For example, when a blue LED is used as the exaction source, if the light source device requires blue light as a color component of the output light, a separate blue LED source needs to be added, and a light collimating device and light combination device need to be added to combine this blue light with the other color components of the output light. This will increase the cost, size and complexity of the light source.